Fluid dispensers are commonly used in the electronics industry to selectively dispense minute amounts or droplets of a highly viscous fluid material onto an electronic substrate, such as circuit board. The types of dispensers commonly used for this purpose include needle, spray, and jetting dispensers. The dispensing valve, or dispenser, that applies the fluid materials is typically part of a dispensing assembly that includes a camera and a height sensor. To dispense fluid materials, the dispensing assembly is moved by a robot, such as an x-y positioner, in a pattern across a surface of the circuit board that bears the components. To provide accurate dispensing, both the horizontal position on an x-y grid and the height (z) of the dispenser must be accurately known so that fluids are dispensed at desired locations on the circuit board, and to prevent unwanted contact between the dispenser and circuit board components. Thus, it is necessary to know the position of the dispenser relative to the locations on the circuit board where the fluid material is to be deposited.
X-Y Offset Calibration Using Contact Height of the Prior Art
When dispensing droplets of material such as adhesive onto a substrate to attach components to the substrate, or underfill components on the substrate, it is necessary to know the x-y-z positions of the components on the substrate, or more precisely, the x-y-z positions of the surfaces on the substrate, or on the components, where material is to be dispensed. To determine these x-y-z positions, a camera and a height sensor ride along with the dispenser on the x-y robot in the system. The x-y positions of the components are then determined based on images from the camera, and the z positions are determined from a height sensor as the camera and height sensor are moved across the substrate. The dispenser is carried on a z-head that moves the dispenser up-and-down.
More specifically, the system uses the camera produce an x-y map of the components by moving the camera over the components on the substrate. The system then likewise moves the height sensor over the components of the substrate and, together with the camera-height sensor offset (described below), produces an x-y-z topographical map of the part/substrate. Given this topographical map, and the camera-dispenser valve offset (described below), the system can dispense at the desired x-y locations and the desired distance z above the substrate. By dispensing at the desired distance z above the substrate, the system can ensure that the dispenser does not contact any of the components on the substrate and travels at the proper process height for dispensing.
Referring to FIG. 1, the camera-dispenser offset is determined as follows. An area known as a “service station” is provided off to the side of the x-y robot where calibration and other functions are performed. The service station has a calibration surface 1 that is used to calibrate the position of the dispenser. The dispensing assembly is moved over the calibration surface 1 to an x-y position 2, and the x-y coordinates of the position 2 recorded by recording the encoder counts of the X and Y drives with the dispensing assembly at position 2. For example, the X drive encoder count might be 500 and the Y drive encoder count might be 1000 so that the (x, y) coordinates of position 2 would be (500, 1000). The dispenser then dispenses a droplet of material onto the calibration surface 1 with the dispensing assembly in the x-y position 2. The camera is then moved over the droplet of material so that the crosshairs of the camera are centered on the droplet. The x-y coordinates of the x-y position 3 of the dispensing assembly are then recorded with the camera centered on the droplet. The recorded x-y position 3 with the camera centered on the droplet might be (600, 1200), for example. Thus, the cross-hairs of the camera in the example are spaced some distance in x and y away from the centerline of the dispenser. The difference between the x-y positions 2, 3 of the dispensing assembly with the dispenser in the calibration position and the camera in the calibration position, using the camera as a reference position, is a camera-dispenser offset vector 4. In this case, the camera-dispenser offset vector 4 would be (500-600, 1000-1200) or (−100, −200). Each encoder count might be equal to 1 mm in distance, for example. Therefore, in this example, the dispenser is located 100 mm to the left of the camera on the X axis and 200 mm “below” the camera on the Y axis, so the offset vector is (−100 mm, −200 mm).
Referring now to FIG. 2, the camera-height sensor offset vector can be determined as follows. In the past, a contact type height sensor was used which had a small probe that contacted the substrate to determine the height of the substrate. U.S. Pat. No. 6,955,946 describes a prior art contact type height sensor at column 4, lines 48-64. With this type of height sensor, the dispensing assembly is moved to position the height sensor probe above the calibration surface 1. The height sensor is then lowered to make an indentation, or calibration mark in a deformable solid on the calibration surface 1, and the x-y position 5 of the dispensing assembly is recorded. For example, the position 5 of the dispensing assembly during formation of the calibration mark may be determined to be (800, 1600). Then, the camera is moved over the calibration mark so that the crosshairs of the camera are centered on the calibration mark, and the x-y position 6 of the dispensing assembly recorded. In this example, the x-y position 6 of the dispensing assembly when the camera is aligned with the calibration mark is (1100, 1700). The difference between the x-y positions 5, 6 of the dispensing assembly with the height sensor in the calibration position and the camera in the calibration position, again using the camera as the reference point, is (800-1100, 1600-1700,) or (−300, −100), which is the camera-height sensor offset vector 7.
Referring now to FIG. 3, in some systems it may also be desirable to know the offset vector between the height sensor and the dispenser, or height sensor-dispenser offset vector 8. The height sensor-dispenser offset vector 8 can be determined from the camera-dispenser offset vector 4 and the camera-height sensor offset vector 7 by determining the difference between the vectors 4, 7. For example, having determined that the camera-dispenser offset vector 4 is (−100, −200), and the camera-height sensor offset vector 7 is (−300, −100), the offset vector between the height sensor and the dispenser, using the dispenser as a reference, can be determined from those two offset values as (−300-(−100), −100-(−200)) or (−200, 100). The x-y-z topographical map, which includes coordinates of all the component locations that were determined using the camera and the height sensor as previously described, is then used with the camera-dispenser offset vector 4, and optionally the height sensor-dispenser offset vector 8, to dispense materials at desired locations on the substrate and from the proper height above the substrate.
X-Y Offset Calibration Using Laser Height Sensor of the Prior Art
It was eventually determined that using a height sensor which makes contact with the substrate is undesirable, so more recently noncontact height sensors such as laser height sensors have been used in the art. Referring now to FIG. 4, the laser height sensor 9 uses a triangulation method to determine substrate height. To this end, a laser beam 11 is projected down to the target surface 13. A beam 15 is then reflected back to the sensor 9 from the target surface 13. The reflected beam 15 is focused through a receiver lens 17 and projected onto a Charge Coupled Device (CCD) 19 within the sensor 9. The CCD 19 detects the peak value of the laser light distribution of the reflected beam 15 for each pixel and determines the precise target 13 position. As the target displacement changes relative to the sensor 9, the position of the reflected beam 15 changes on the CCD 19. Measurement of the position of the reflected beam 15 on the CCD 19 thereby provides stable and accurate height sensing on a variety of target surface types.
To determine the camera-height sensor offset vector when a laser height sensor is used, a calibration mark is placed on the service station calibration surface, and the laser beam spot is manually (i.e. visually) centered on the calibration mark. After the laser height sensor has been manually positioned in this way, the (x, y) coordinates of the sensor are recorded and the camera-height sensor offset vector is determined in the same way as described above for a contact height sensor. However, this method has two problems. The first problem is that relying on an operator to manually position the laser spot on the calibration mark introduces human error in that one must visually judge when the laser spot, which is very tiny, is centered on the calibration mark. This can result in errors of a hundred microns or more off of the true center of the calibration mark. Given the high level of precision needed in the electronics industry when dispensing tiny dots of material on small components, being 100 microns or more off target when dispensing a dot of material can be unacceptable. The second problem is that the current method is stressful on the eyes of the operator given that the operator must look at a bright laser light. This stress on the operator can cause even more inaccuracy when visually positioning the laser on the calibration mark.
Therefore, improved apparatuses and methods for determining the position of a noncontact height sensor, such as a laser height sensor, are needed. These improved apparatuses and methods should be more accurate and less stressful on the operator, or remove the need for an operator to perform the noncontact height sensor alignment task. By positioning the height sensor more accurately, a more accurate topographical map of the components on the substrate can be created, which will enable the system to more accurately dispense materials onto the substrate.